Your brain stopped making new cells at age 13, study claims

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Photos:Neurodegenerative diseases: The latest stories from CNN

College freshman Indrani Das hit headlines in 2017 for her research into brain cells and preventing the damaging side effects that occur when the brain heals. For people with brain injuries and neurodegenerative diseases like Alzheimer's, her findings could one day help them live a better life.

Scroll through to discover more of CNN's coverage of neurodegenerative disease research.

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Photos:Neurodegenerative diseases: The latest stories from CNN

A simple walking test to diagnose form of dementia – A study published February 2018 stated that walking while counting backwards was an accurate way to differentiate between idiopathic normal pressure hydrocephalus (a condition that affects elderly mobility, which may sometimes be reversed with early treatment) from a form of dementia called progressive supranuclear palsy. The two are often confused by doctors, but a pressure-sensitive carpet and detailed study of a patient's gait demonstrated key differences between the two conditions. Read more

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Photos:Neurodegenerative diseases: The latest stories from CNN

Excessive alcohol consumption linked to early-onset dementia – A February 2018 study looked at over one million adults diagnosed with dementia released from French hospitals between 2008 and 2013. It found a diagnosis of alcohol-use disorders ("the chronic harmful use of alcohol or alcohol dependence") among 16.5% of male subjects and 4% of female subjects with dementia -- more than twice the levels of those without dementia. There was a particularly strong association for those with early-onset dementia. Read more

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Tests suggests protein linked to Alzheimer's – A study released in February 2018 claims scientists in Australia and Japan have developed a new blood test to detect the early stages of Alzheimer's disease. The noninvasive test was developed to detect the presence of the toxic protein amyloid beta, known to be present in people affected by the disease, and did so with 90% accuracy, according to the paper. Read more

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Photos:Neurodegenerative diseases: The latest stories from CNN

New drug trial shows promise in fight against Huntington's – In December 2017 an experimental drug was shown to reduce levels of the toxic protein that causes Huntington's disease, an inherited disorder in which mutated proteins damage nerve cells in the brain. The new drug, Ionis-HTTRx, was shown to be safe for humans in a trial of 46 patients and was described as a "potential game-changer." Read more

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Photos:Neurodegenerative diseases: The latest stories from CNN

Bill Gates steps into the Alzheimer's fight – In November 2017, tech billionaire and philanthropist Bill Gates announced a $50 million investment in the Dementia Discovery Fund, a research partnership focused on what drives brain disease. "Several of the men in my family have this disease (Alzheimer's)," Gates told CNN's Dr Sanjay Gupta. "I've seen how tough it is. That's not my sole motivation, but it certainly drew me in." Read more

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A way to test for Alzheimer's in living patients? – In September 2017, a study was published outlining an experimental blood test capable of diagnosing Alzheimer's with 86% sensitivity and specificity. (Sensitivity refers true positives identified by the test, while specificity refers to true negatives.) Read more

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Is there a link to poor sleep? – A study in the journal Neurology dated September 2017 suggested people who get less REM (dream-stage sleep) may be at higher risk for developing dementia. The study found that people who took longer than the typical 90 minutes to enter REM were more likely to get dementia. Read more

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Neurodegenerative disease found in 99% of studied brains from deceased NFL players – Chronic traumatic encephalopathy, known as CTE, was found in 99% of deceased NFL players' brains that were donated to scientific research, according to a study published in July 2017. The neurodegenerative brain disease can only be formally diagnosed with an autopsy at present. The study points out potential bias because relatives of these players may have submitted their brains due to clinical symptoms they noticed while they were living.Read more

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FDA approves first new drug for ALS treatment in 22 years – In May 2017 the US Food and Drug Administration approved Radicava to treat patients with amyotrophic lateral sclerosis (ALS) -- the first new drug approved to treat the neurodegenerative disease in over two decades. The progressive disease affects nerve cells in the brain and spine, causing the gradual loss of muscle movement, leading to paralysis and death. Tests in Japan found patients experienced a smaller decline in daily functioning than those who received a placebo, with the manufacturer saying the drug slowed the decline of physical function by 33%. Read more

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Could Alzheimer's bankrupt Medicare? – A March 2017 report stated that every 66 seconds an American will develop Alzheimer's disease -- and that could reduce to 33 seconds by 2050. The cost for dementia and Alzheimer's care in the US hit $259 billion in 2017, with Harvard professor Rudy Tanzi arguing the extra stresses of baby boomers developing Alzheimer's could "single-handedly collapse Medicare/Medicaid." Read more

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A less painful way to diagnose Parkinsons's disease? – 1% of 60-year-olds have Parkinson's, rising to 4% by the age of 80, per the Centers for Disease Control and Prevention. Traditionally a painful spinal tap was required to differentiate Parkinson's from other similar diseases (an atypical parkinsonism disorder). But a new study was reported in February 2017 to have found a way to distinguish Parkinson's via a blood test. Read more

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An Alzheimer's drug by 2025? – In December 2016, experts said they still hold out hope for a drug to combat the disease by 2025 -- most likely a pilot version that will need to be upgraded. The goal was first outlined at a G8 dementia summit in 2013. CNN profiled a number of studies to tackle the diseases from different angles. Read more

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A way to control Parkinson's beyond drugs? – In February 2016 CNN profiled the Gyroglove, hardware proposed as a drug-free alternative to Parkinson's tremors. Treatments for the disease become less effective over time, and there is no known cure. The GyroGlove uses a disk mounted in the back of the hand which spins at around 20,000rpm, which steadies motion. Read more

Story highlights

(CNN)Your brain has peaked by age 13. The creation of new brain cells in the hippocampus ends before we even reach adulthood, according to a study published Wednesday in the journal Nature.

Previously, scientists generally believed that the human hippocampus generates new brain cells or neurons throughout adulthood. There's no consensus about the number of neurons made in adulthood; some researchers believe that hundreds of neurons are added each day, while others contend that very few are created after childhood.

This birth of new brain cells -- or neurogenesis -- is "a fascinating process," said Arturo Alvarez-Buylla, senior author of the new study and a professor of neurological surgery at the University of California, San Francisco.

What would it mean?

"It requires the birth of the cell, the migration or the movement of the cell to the right place -- which is not an easy task in the very dense structure of the brain -- and then that cell has to make space to grow and connect to other cells and then contribute in a functional way to that circuit," he said.

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Neurons are created by progenitor cells, also called precursor or stem cells. Newborn neurons are important because they have different properties than mature neurons. In particular, they excel at forming new connections.

For the study, the researchers searched for progenitor cells and young neurons in 59 postmortem and postoperative human tissue samples taken from the hippocampus. The hippocampus, a tiny seahorse-shaped structure deep inside the brain, plays a major role in learning and remembering. Alvarez-Buylla's research focuses on one brain region at a time.

The ages of the brain tissue samples ranged from early fetus through adulthood.

In fetal samples that had gestated 14 weeks, the researchers saw rich streams of precursor cells and immature neurons migrating to a still-developing region within the hippocampus known as the dentate gyrus, known to be crucial to memory formation.

Searching additional fetal samples, the researchers discovered that by 22 weeks, the migration of cells had decreased. By age 1, there were far fewer immature neurons than during the fetal stage. Older samples showed even fewer young neurons. Age 13 was the oldest brain tissue sample containing any immature neurons, the researchers found.

The adult tissue samples had none.

What does it mean that our brains do not make new brain cells beyond age 13?

Alvarez-Buylla asks instead, "What would it mean if we did make neurons in this part of the brain?"

Some people believe the process of neurogenesis adds to the rejuvenation of the brain, yet no one knows whether this is the case, he explained.

"If the adult brain contains some stem cells that might be reactivated or rekindled," he wondered, "would it really contribute in an important way to any plasticity or even some recovery from the loss of neurons as happens in some diseases?" It might be good, he said, but it also might not.

René Hen, a professor in Columbia University's departments of psychiatry, neuroscience and pharmacology at the Kavli Institute for Brain Science, said the new study is "provocative."

Harnessing youth

"It is provocative because it claims that in the average human brain there are very few neurons left -- in other words, it's probably not functional," said Hen, who was not involved in the research. "It is important because there are few studies that have documented how many of these young neurons are present in the hippocampus in humans."

He said there's been "a lot of interest in trying to harness these young neurons to improve cognition."

"We don't know how many of these young neurons are important for function," he said. There's evidence that interneurons -- a different type of brain cell -- mediate the function of young neurons and even amplify their effects.

"So even a small number of young neurons could actually have quite potent effects," Hen said. "How many is enough to produce a function is really the question."

Why were no young neurons found in adult brain tissue in this study, when previous research found many or at least some neurons? Hen suggested that the use of tissue samples might explain the discrepancy. "When you work with postmortem human tissue, the tissue is often of poor quality," he said, and so it is "very difficult" to assess the number of neurons contained there.

Hen added that biology also affects neurons, so the postoperative samples might also be problematic.

"In cases of psychiatric disorders, in cases of stress, in cases of inflammation, the number of neurons decreases," Hen said. Future research is needed to reconcile the differences between past studies and this new work, he said. "The jury is still out."

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Jason Snyder, an assistant professor in the department of psychology at the University of British Columbia, described the new research as "important."

Snyder's commentary was published Wednesday in Nature beside the new study, in which he was not involved.

"It provides strong evidence that the human brain's ability to produce newborn neurons in the hippocampus (a brain region involved in memory formation) is limited as we get older," Snyder wrote in an email.

The new study also needs to be taken seriously because it guides us to explore new directions. "If we can understand how neural precursor cells work, we may be able to use them to replace neurons that have died," he said.

Alvarez-Buylla said the most "striking" observation of the paper was something else that was missing, besides the new neurons.

"In the rat brain, there is a very dense layer of cells, some of which are actively proliferating, and this is where the progenitors of the new neurons meet. This region is called the granular zone," he said. "That structure was simply not there in humans."

Clarification: A previous version of this story did not state Arturo Alvarez-Buylla's affiliation with University of California, San Francisco.